When air is sucked into a cylindrical basket-shaped centrifugal fan of a multiblade centrifugal blower, because inertial force acts on the sucked-in air, the sucked-in air flows towards the hub of the centrifugal fan. There is a space between neighboring blades of the centrifugal fan (hereinafter, “blade space”). Due to the flow of the sucked-in air towards the hub, a reverse flow of air is created inside the centrifugal fan in the space between the blade space that is close to the air inlet, and the reverse flow of the air blocks the main flow of the sucked-in air. Furthermore, a secondary flow of air is created along an inner wall inside a scroll casing of the centrifugal fan causing deterioration in performance of the multiblade centrifugal blower.
A conventional multiblade centrifugal blower (for example, see Patent Document 1) includes a centrifugal fan and a scroll casing that houses the centrifugal fan. The centrifugal fan rotates around a rotation shaft, which extends vertically, sucks in air from the direction of the rotation shaft, and blows the sucked-in air in a circumferential direction. The scroll casing includes an air inlet on an upper wall, a casing main body with a spiral air passage around the centrifugal fan, and a cylindrical exhaust. The cylindrical exhaust, which protrudes from a peripheral part of the casing main body, communicates with the air passage and forms a tongue in the peripheral part of the casing main body. Near the tongue formed in the scroll casing, a rectifying rib is arranged to guide the air, which is likely to flow in a reverse direction from a space between the top part of the centrifugal fan and the scroll casing towards the air inlet, in blowing direction. This configuration leads to reduction in noise.
Another conventional multiblade centrifugal blower includes a centrifugal fan and a scroll casing (for example, see Patent Document 2). The centrifugal fan includes a hub that rotates around a shaft center, a plurality of blades that are arranged at predetermined intervals in a circumferential direction of the hub, and a ring located on the opposite side of the hub to reinforce the blades. The scroll casing includes an air inlet, and houses inside the centrifugal fan such that the centrifugal fan is rotatable. A depressed portion of a predetermined depth, that is, a bell mouth is arranged circumjacent to the air inlet of the scroll casing. The air inlet-end located on the opposite side of the blades on the hub does not have shroud, and is rotatably inserted inside the recess of the bell mouth. Thus, noise is reduced in the conventional multiblade centrifugal blower.
Patent Document 1: Japanese Patent Application Laid-Open No. 2004-245087
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-353665